1. Field of the Invention
The present invention relates to an anode thin film for lithium secondary battery and a preparation method thereof, and more particularly, to an anode thin film for a lithium secondary battery having a reduced initial irreversible capacity by using an intermetallic compound of tin (Sn) and nickel (Ni) as a material of forming an anode active material layer formed on a current collector, and having improved charging/discharging cycle characteristics by solving the problem of aggregation of tin due to intercalation/deintercalation of lithium.
2. Description of the Related Art
It is known that metallic lithium in an organic electrolyte is thermodynamically unstable and is coated with a thin surface layer called a solid electrolyte interface (SEI). The unstable property of the metallic lithium used as an anode-forming material may result in dendrite growth during repeated cycles of charging and discharging, leading to deterioration in stability of battery.
A lithium ion battery can secure its stability and can maintain high capacitance characteristics by employing graphite as an anode-forming material. A graphite anode reversibly enables storage and separation of lithium through intercalation of lithium ions. Since the inserted lithium ions are not in the form of metal, factors that may adversely affect the stability of a battery, for example, growth of dendrite, can be prevented.
However, graphite anodes have only approximately 10% of an energy density of metallic lithium anodes. Various anode materials including disordered carbon, nitrides or oxides, which exhibit improved capacity compared to graphite, have been proposed for overcoming the problem of small energy density.
Various studies Oxide-based anodes exemplified by tin oxides were proposed for the first time by Fuji Photo Film Co., Ltd., in U.S. Pat. No. 5,618,640 by Y. Idota et al, and in a paper by Idota et al. entitled “Tin-Based Amorphous Oxide: A High Capacity Lithium-Ion Storage Material”, Science, 276 (1997) 1395-1397, and have approximately twice a charge/discharge capacity of graphite-based anodes and are excellent in potential characteristics compared with existing substitutes.
Oxide-based anodes consist of oxides of metal that can form lithium alloys, such as tin oxide (SnO or SnO2), lead oxide (PbO) or silicon oxide (SiO). These oxide-based anode-forming materials serve as precursors of anode active materials. Thus, as lithium ions are diffused across the lattice of metal, oxygen ion and metal ion are separated from each other. Also, as intercalated lithium ions react with oxygen ions, separation occurs in the lattice of metal. Active materials capable of charging/discharging are substantially metals separated in the above-described manner, rather than metal oxides.
It is also known that lithium can be stored/removed by an alloying reaction between tin and lithium.
The oxide-based anodes consisting of oxides, e.g., tin oxide, have good cycle characteristics, compared with lithium alloys, in the following reasons.
First, as lithium ions are diffused across the lattice of metal, a metal that can form lithium alloys, e.g., tin, is separated. However, the separated metal is very small in size, the mechanical loss of an active material, due to a change in volume, can be minimized.
Second, since, during initial stage of lithium storage, lithium oxide (Li2O), which is a product based on the reaction between the lithium produced simultaneously with separated metal during initial storage of lithium, and the oxygen ions, is evenly dispersed, damage of the active material due to a change in volume can be suppressed.
However, the oxide-based anode exhibits a relatively high initial irreversible capacity due to formation of lithium oxide, which is necessarily generated during the first charging/discharging cycle. Accordingly, an excess amount of a cathode active material is required, which is impediment to practical use.
In order to reduce the initial irreversible capacity while maintaining high capacity and good cycle characteristics of an oxide-based anode, there have been proposed methods in which an intermetallic compound or nano-sized metal powder is employed. The methods in which an intermetallic compound is employed are disclosed in U.S. Pat. No. 6,203,944 by Robert L. Turner et al, entitled “Electrode for a lithium battery,” papers entitled “Mechanically Alloyed Sn—Fe(—C) Powders as Anode Materials for Li-Ion Batteries,” by Mao et al., J. Electrochem. Soc., 146 (2) (1999) 405-413, “The Reaction of Lithium with Sn—Mn—C Intermetallics Prepared by Mechanical Alloying,” by Beaulieu et al, J. Electrochem. Soc., 147 (9) (2000) 3237-3241, and “LixCu6Sn5 (0<x<13): An Intermetallic Insertion Electrode for Rechargeable Lithium Batteries,” by Kepler et al., Electrochem. Solid-State Left., 2 (7) (1999) 307-309. The methods in which nano-sized metal powder is employed are disclosed in papers entitled “Sub-Microcrystalline Sn and Sn—SnSb powders as Lithium Storage Materials for Lithium Ion Batteries,” by Yang et al., Electrochem. Solid-State Lett., 2 (4) (1999) 161-163, and “Ultrafine Sn and SnS0.14 Powders for Lithium Storage Materials in Lithium-Ion Batteries,” by Yang et al., J. Electrochem. Soc., 146 (11) (1999) 4009-4013.
The former methods will now be described in more detail. A tin-based intermetallic compound, for example, Sn2Fe or Cu6Sn5, consists of an intermetallic compound of a metal which does not form a lithium alloy and a metal which is reactive with lithium. Also, since the tin-based intermetallic compound does not undergo irreversible reaction, e.g., formation of lithium oxide (Li2O) due to diffusion of lithium ions into the lattice of metal, unlike tin oxide, initial irreversible capacity can be reduced.
However, the tin-based intermetallic compound causes aggregation of tin due to repeated intercalation/deintercalation of lithium ions, which aggravates the mechanical damage of an active material depending on a change in volume, like metallic tin, resulting in considerable deterioration of cycle characteristics.
To solve the above-described problems, an attempt to use a composite material of an active-phase material enabling intercalation/deintercalation of lithium and an inactive-phase material non-reactive with lithium, prepared by a mechanical alloying method, as an anode forming material, has been made.
Detailed examples of the composite material include a composite material consisting of Sn2Fe as an active-phase material and SnFe3C as an inactive-phase material. While such a composite material has a fine structure, improved cycle characteristics due to addition of inactive-phase material, and an increased energy density per volume, its energy density per weight is very small, i.e., less than 200 mAh/g.
As shown in FIG. 1, a lithium-tin alloy (Li4.4Sn) has a relatively low operating voltage with respect to a lithium electrode, i.e., 0.7 V or less, and has an energy density per unit weight of approximately 790 mAh/g, which is higher than that of a lithium-graphite compound (LiC6) having an energy density of 342 mAh/g.
In the lithium-tin alloy enabling intercalation/deintercalation of lithium, aggregation of tin, which is due to intercalation/deintercalation of lithium, and a severe change in volume, cause cracks on the surface of and within tin, which leads to electrical disconnection with a current collector, thereby deteriorating cycle characteristics, which is confirmed in FIG. 2.
Referring to FIG. 2, when an anode of lithium-tin alloy film is charged/discharged with a constant current of 50 μA/cm2 up to 0 to 1.2 V, the initial charge capacity thereof began to sharply decrease after repetition of 5 cycles until it vanishes after repetition of 20 cycles, that is, the cycle life is deteriorated.